How to Play
Drag the counter onto the board. Each counter adds the value shown for that slot.
Red counters show the numeral subtracted from the blue numeral.
A subtraction-pair slot adds both its red and blue counters. Click any placed counter to remove it.
How Roman Numerals Work
Roman numerals use letters instead of digits to show numbers.
- I means 1
- L means 50
- V means 5
- C means 100
- X means 10
- D means 500
- M means 1,000
Subtraction Rule
If a smaller numeral comes before a bigger one, subtract it.
IV = 4 one before five
IX = 9 one before ten
XL = 40 ten before fifty
Addition Rule
If a smaller numeral comes after a bigger one, add it.
VI = 6 five plus one
XI = 11 ten plus one
XV = 15 ten plus five
Try using the counters to build numbers and see how the Roman numerals are made.